The role of nuclear quantum effects in the relative stability of hexagonal and cubic ice

At atmospheric pressure, hexagonal ice (I_h) is thermodynamically stable relative to cubic ice (I_c), although the magnitude and underlying physical origin of this stability difference are not well defined. Pure I_c crystals are not accessible experimentally, and hence computer simulations have often been used to interrogate the relative stabilities of I_h and I_c; however, these simulations are dominated by molecular interaction models that ignore the intramolecular flexibility of individual water molecules, do not describe intermolecular hydrogen-bonding with sufficient accuracy, or ignore the role of nuclear quantum effects (NQEs) such as zero-point energy. Here, we show that when comparing the relative stability of I_h and I_c using a flexible, anharmonic molecular interaction model, while also accurately accounting for NQEs, a new picture emerges: I_h is stabilized relative to I_c as a result of subtle differences in the intramolecular geometries and intermolecular interactions of water molecules which are modulated by NQEs. Our simulations hence suggest that NQEs are a major contributor to the stabilization of I_h under terrestrial conditions and thus contribute to the well-known hexagonal (sixfold) symmetry of ice crystals.